Synthesis of CuCo2S4@Expanded Graphite with crystal/amorphous heterointerface and defects for electromagnetic wave absorption

The remarkable advantages of heterointerface and defect engineering and their unique electromagnetic characteristics inject infinite vitality into the design of advanced carbon-matrix electromagnetic wave absorbers. However, understanding the interface and dipole effects based on microscopic and macroscopic perspectives, rather than semi-empirical rules, can facilitate the design of heterointerfaces and defects to adjust the impedance matching and electromagnetic wave absorption of the material, which is currently lacking. Herein, CuCo2S4@Expanded Graphite heterostructure with multiple heterointerfaces and cation defects are reported, and the morphology, interfaces and defects of component are regulated by varying the concentration of metal ions. The results show that the 3D flower-honeycomb morphology, the crystal-crystal/amorphous heterointerfaces and the abundant cation defects can effectively adjust the conductive and polarization losses, achieve the impedance matching balance of carbon materials, and improve the absorption of electromagnetic wave. For the sample CEG-6, the effective absorption of Ku band with RLmin of −72.28 dB and effective absorption bandwidth of 4.14 GHz is realized at 1.4 mm, while the filler loading is only 7.0 wt. %. This article reports on the establishment of potential relationship between crystal-crystal/amorphous heterointerfaces, cation defects, and the impedance matching of carbon materials.


Supplementary equations
(1) Magnetic loss of composite materials is generally caused by natural resonance, exchange resonance and eddy current effect.In order to further analyze the type of magnetic loss, C0 is introduced and the equation is as follows: The 0 for vacuum magnetic permeability, for conductivity and frequency.
(2) According to the theory of debye ″ depends on the conduction loss ( ″ ) and polarization loss ( ″ ) comprehensive contribution.Its equation is as follows: Among them, the ω for angular frequency, σ for electrical conductivity, 0 for the vacuum dielectric constant (8.85 × 10 −12 Fm −1 ), as the static dielectric constant, ∞ for infinite frequency dielectric constant, τ as relaxation time.
(3) In order to study the polarization relaxation process, uses the ´-″ curve to describe the cole-cole semicircle.According to the theory of Debye ´and ″ relationship can be expressed in the type.
(5) (4) The normalized input impedance () is expressed by the equation, as following: Where, Zin is the input characteristic impedance, Zo is the free space impedance, and d is the material thickness.
treatment, the diffraction intensity of graphite peak decreased significantly, and the expanded graphite peak shifted to the right.Supplementary Figure 3b shows the This is significantly different from EG prepared by the conventional method and Humer method.The intercalator and functional group change information before and after EG preparation were characterized by FTIR, as shown in Supplementary Figure 3c.In all samples, the absorption band at 3430cm -1 was attributed to stretching vibrations of -OH.The band at 1620cm -1 corresponds to C=C vibration.The band at 875cm -1 is attributed to C-O-C symmetry and stretching.However, new vibration peaks appear at 1240-1257cm -1 and 583-589cm -1 of expandable graphite.This is caused by the asymmetric stretching vibration of S=O and S-O.The results show that sulfuric acid and sulfate can be intercalated into graphite quickly under the condition that the graphite is not seriously caused by microwave.
In addition, XPS was used to further detect the surface chemical composition or surface functional groups of expanded graphite, as shown in Supplementary Figure 3d-g  Supplementary Table 3 Raman spectrum and the intensity ratio of D band to G band (ID/IG) of EG before and after microwave-assisted preparation.It can be observed from the Figure that there are two obvious strong absorption peaks G and 2D at 1581cm -1 and 2720 cm -1 , respectively.This indicates that EG before and after microwave-assisted treatment has very regular structure and large graphene domain size.Meanwhile, the D peak at 1338cm -1 was observed in the Raman spectrum.The D-peak of microwave-assisted EG disappeared almost completely, and the ratio of D-peak to G-peak was only 0.06.

Table 2 .
. In the range of 100eV~700eV, EG mainly shows characteristic peaks related to C 1s, O 1s and S 2p.After microwave treatment, the C/O ratio of graphite decreased from 14.79 to 3.78 and then increased to 15.86.The results show that microwave irradiation can effectively reduce the C-O, C=O oxygen-containing groups covered by expanded graphite surface, which is beneficial to obtain high-performance three-dimensional carbon-based conductive network.As shown in Supplementary Figure3h and i, the prepared expanded graphite has good thermal and electrical conductivity.This provides a new route and raw material supply for the preparation of high-performance carbon based microwave absorbing materials.EXAFS fitting parameters at the Cu and Co K-edge for various samples.

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Comparison with the heat dissipation properties previously reported for similar carbon materials.